Mak, Margaret K.Y. PhD
Children with developmental coordination disorder (DCD) demonstrate below age-level performance in skilled and coordinated tasks despite normal intelligence and no neurological abnormalities.1 Reach-and-grasp is frequently performed in play activities, but it is one of the most common skills with which children with DCD experience difficulties.2 Ninety percent of children with DCD performed particularly poorer in the “upper limb coordination” subtest of Bruininks-Oseretsky Test of Motor Proficiency3 and Movement Assessment Battery for Children (MABC).4,5 Poor performance in these tests can lead to poor sport performance. Consequently, children with DCD rarely participate in sport activities and ball games, which can hinder their physical and social development, which in turn could lower their perceived global self-worth.
Previous studies used reaction time (RT), movement time (MT), and peak force (PF) to examine motor problems in children with DCD. Reaction time provides information about the speed and accuracy of sensory information processing, translation of that information into an action plan, and the initiation of an overt response.6 Using simple and choice RT paradigms, researchers have found that children with DCD have longer RTs to press a switch with the thumb in response to a proprioceptive stimulus.7,8 Children with DCD were reported to have significantly longer RTs to release a “start key” after seeing a green arrow on a computer screen.9 The prolonged RT suggested that children with DCD had slowness in processing proprioceptive and/or visual information.7–9 In addition to RT, Henderson et al9 found that children with DCD had longer MTs than subjects who were developing typically to reach their arm forward to press a targeted key indicated by a green arrow. Variability of MT was also significantly greater in children with DCD than in children who were developing typically.9,10 A few studies investigated force control in static tasks in children with DCD.11,12 Pereira et al.11 found that children with DCD used excessive PFs when lifting static objects of different texture and weight with a precision grip. Other investigators reported that children with DCD could reach the targeted force but had difficulties to maintain a steady force.11,12
Published data hitherto examined time control during performance of simple tapping or aiming tasks or investigated PF used to grip static objects in children with DCD.13 These findings, however, could not be generalized to reaching and grasping fast-moving objects that required an interaction of speed and force control. An understanding of motor control in the performance of this functional goal-directed task is useful in designing treatment interventions to improve this action, which is commonly found in play activities. Therefore, the present study aimed to investigate the performance of a reach-and-grasp task in children with DCD. Specifically, we examined whether these children (1) had longer RTs and MTs and generated larger PFs than control children when reaching out their hand to grasp a moving target and (2) could adjust their RT, MT, and PF in response to a change in the speed of the target.
The study included 16 children (mean age = 8.1 ± 0.6 years) diagnosed by a pediatrician to have DCD according to the Diagnostic and Statistical Manual of Mental Disorders1 and with a total impairment score on the MABC test below the 15th percentile (Table 1).5 Eleven children (mean age = 7.9 ± 0.6 years) who were reported by caregivers as not having any gross motor functional problem were recruited from local community centers as the control group. Children who had limited range of upper limb movement, visual problem, or difficulties in understanding and following instructions were excluded. Independent t tests and χ2 tests indicated no significant between-group difference for age and gender, respectively (Table 1). The experimental procedures were approved by the University Ethics Committee. Informed consents were obtained from participants and their parents in accordance with the 1964 Declaration of Helsinski.
A toy car that weighed about 180 g was used as the reach-and-grasp target. The car was allowed to slide down a slanted board, which had an adjustable slope of 8° or 15°. Figure 1 shows the instrumental setup. Children sat on a chair, facing the slope with their dominant thumb pressed on a button at the bottom of the slope (Figure 2a). Initially, the car was pressed against a button at the top of the slope by a circuit that activated a magnet (Figure 2b). When the circuit was disconnected, the car was allowed to slide down the slope. Children were instructed that once they saw the car sliding down the slope, they had to release their dominant thumb from the bottom button, reach and grasp the sides of the toy car with this hand, and lift it off the slope as soon as possible (Figure 2c). The car was slid down a slope of 8° (top speed of the toy car = 0.89 m/s) or 15° (top speed of the toy car = 1.07 m/s).
Children were allowed to have 1 practice trial for each condition (8° slope, 15° slope) prior to 5 test trials. Both the slope and test trials were arranged in a randomized manner. RT, MT, and PF were calculated (Figure 3). To register the force applied in grasping the toy car, a load cell (with a measurement accuracy of 98%) was located inside the toy car. A programmable input-output controller was attached to the top and bottom buttons of the slanted board as well as to the load cell inside the toy car to record RT, MT, and PF (Figure 1f). All test trials were videotaped to review the performance of the reach-and-grasp task for each child. If a child did not pick up the toy car in the correct manner or if the toy car ran off the slope without being picked up by the child, those test trials were classified as failed trials. The failure rate was expressed as a percentage of the number of failed trials over the total test trials in each subject group.
Each child was required to perform 5 test trials for each condition (8° slope and 15° slope). The control group completed a total of 110 test trials, and the DCD group completed 160 trials. The failed trials were coded according to the errors made during the reach-and-grasp task. Descriptive statistics were used to analyze the data of the failed trials.
For each condition, the mean values of the 5 test trials of each child were used for data analysis. RT, MT, and PF were analyzed using 2-way repeated-measures analysis of variance (ANOVA). The between factor was group (DCD and control), and the within factor was slope (8° and 15°). When an interaction was found, post hoc tests were used to determine the real difference. Independent t tests were used to analyze the differences between the DCD group and the control group for the percentage changes in RT, MT, and PF with respect to an increase from an 8° to a 15° slope. Spearman correlation was performed to establish the relationship between MABC score of the children with DCD and RT, MT, and PF. A significance level of .05 was used for all analyses.
Children in the control group successfully performed the reach-and-grasp task in 110 test trials. Among the total 160 test trials, children in the DCD group failed 56 trials (35%). The failure rates for the 8° and 15° slope were similar, which were 17% (27 trials) and 18% (29 trials), respectively (Figure 4a). The causes of the failure were as follows: “The child picked the toy car up at the wrong spot,” followed by “The child pressed down the toy car to stop it rather than picked it up as instructed,” and “The toy car ran off the board without being picked up by the child.” Hence, the major cause of failure in children with DCD was the execution of faulty motor strategies rather than that they were too slow to complete the task. Figure 4b further illustrates that the number of failed trials was similar, especially from test trial 1 to 3 in each condition.
When the toy car was slid down the 8° slope, children with DCD had significantly longer RTs (by 17.5%, P = .017) and significantly longer MTs (by 5.8%, P = .042) and used significantly larger PF (by 28.5%, P = .001) than control children (Table 2 and Figure 5). When the slope increased from 8° to 15°, there was no significant change in RT in both groups. However, children in both the DCD and the control groups shortened MT and increased PF significantly (P < .001). The insignificant Group × Slope interaction implies that both groups made similar changes in MT and PF with respect to the changes in the angle of the slope. At the 15° slope, significant between-group difference for RT, MT, and PF remained. The Spearman correlation coefficient showed that there was no significant association between the MABC score and RT, MT, and PF (r = −0.194 to 0.370, P > .05).
Our key findings showed that children with DCD were less efficient than the control children when reaching and grasping a moving target. Children with DCD only successfully completed 65% of test trials, whereas control children had a 100% success rate. Within the successful trials, children with DCD had significantly longer RTs and MTs as well as larger PF than control subjects when reaching and grasping a moving toy car. However, when the angle of the slope increased from 8° to 15°, children with DCD adjusted their MT and PF in a similar manner as that of control children. Children with DCD appeared to have the ability to modify their ongoing movement in response to an increase in the speed of a moving target.
Children with DCD made more errors in performing this dynamic reach-and-grasp task. This finding agreed with previous findings that these children made more absolute errors in generating a target force13 or more spatial errors when aiming at a static target.14 Note that in the present study, more than 80% of the failed trials resulted from executing inaccurate motor strategies; that is, they picked the toy car up at the wrong spot or pressed down the toy car to stop it rather than picking it up. Children with DCD were proposed to have high noise level in their motor system,15 resulting in more variance in the motor output than control subjects. It is possible that under the time constraint of reaching and grasping a moving target, the noise level would have increased in children with DCD and they elicited faulty motor responses.
Reaching and Grasping a Toy Car Sliding Down an 8° Slope
Previous data showed that children with DCD had prolonged RTs in response to static visual stimuli such as a green arrow on a computer screen9 or a light flash.16 We found that children with DCD had significantly longer RTs than control subjects to release a button when they detected that the toy car started to move. Results of previous studies and the present study imply that children with DCD could have delay in detecting static or moving visual stimuli. In addition, the previous finding of a prolonged premotor time found in children with DCD suggested that these children could have delays in central processing, that is, time taken for stimuli registration, coding, processing, and response programming.13 In the present study, the prolonged RT in children with DCD could be related to their difficulties in detecting the moving car, processing the visual-spatial information of the car, and/or planning for the reach-and-grasp action.9,17
The significantly longer MT found in children with DCD to reach and grasp the moving toy car was consistent with previous findings of prolonged MT to aim at a moving target18 and to reach and grasp a static target.19 The prolonged MT could be due to impaired feed-forward and/or kinesthesia control. During the performance of the reach-and-grasp task, visual information about the position of the car could be used as feed-forward information to preplan the action, or as feedback providing ongoing correction to enable the action to be performed in accurate speed, direction, and distance. Children with DCD could have problems in processing visual-spatial information from the moving toy car to plan this fast and accurate reach-and-grasp task.9,17,20 The deficits in executing an anticipatory strategy would force children with DCD to rely heavily on visual information as feedback for movement correction.21 Hence, the movement speed would be slowed. During the “grasp” action, subjects have to preshape their hand to grip and pick up the moving car from the slope. Children with DCD were reported to have weaker sensitivity to proprioceptive inputs.8,12 These children might have required a longer time to perceive the proprioceptive information about the position of the fingers and to refine the hand grip to pick up the car. Future studies examining the acceleration and deceleration phases of the forward-reach component as well as the sensorimotor interaction of the grasping action are needed to elucidate these postulations.
For PF, children with DCD produced a significantly greater force to grasp a moving toy car (P = .001, Table 2) than control subjects. No study has examined the force control of gripping a moving target in children with DCD. Pereira et al12 reported that children with DCD used excessive isometric finger-tip PF to lift up static objects. The larger PF exerted by children with DCD to secure their grasp of the moving toy car could be a compensatory strategy for their poorer kinesthetic perception.8,12 The increased PF in children with DCD could also be due to delayed onset of antagonist muscle activity and/or prolonged agonist muscle activity. Future study employing electromyography could confirm this hypothesis.
Reaching and Grasping a Toy Car Sliding Down a 15° Slope
In the present study, both control children and children with DCD did not modify their RT in response to the change in the angle of the slope. It was possible that the reach-and-grasp task was simple; therefore, no extra time was required to plan the movement strategy despite a variation in the speed. In contrast to RT, children in both the control and the DCD groups made significant changes in MT and PF when the slope increased from 8° to 15° (Table 1). The most intriguing finding was that children with DCD could modify their MT and PF in a similar manner to that of control children, suggesting that children with DCD were able to use the visual-spatial information (ie, the change in speed of the toy car) as feedback to modulate their ongoing movement. However, our finding was in contrast to previous data. Children with DCD were found to be less affected by visual feedback distortion in a center-out drawing task than control children, suggesting that visuomotor adaptation operated differently in children with DCD.22 Mon-Williams et al19 reported that children with DCD could not use visual cue to modify their movement trajectories when reaching to new target positions. Children with DCD could have difficulty in making an online correction of movement direction.19 Modification of MT, on the other hand, could be simpler than that of movement direction and therefore within the ability level of children with DCD. For PF, Smit-Engelsman et al13 reported that children with DCD could adjust an isometric force with their index finger to match a visual target. Even under time pressure as shown in the present study, children with DCD could modulate their peak grip force to secure a faster-moving target.
The present study had a number of limitations. First, the present study did not employ kinematic movement analysis of the reach-and-grasp task. Therefore, the trajectory, velocity, and acceleration pattern of the upper limb could not be examined. Second, the present study included a small sample of children with DCD, and the number of the control subjects and subjects with DCD was not equal. Results of the study can not be generalized to children with DCD of all ages and with different disability levels.
To conclude, this is the first study showing that reaching and grasping a moving object is impaired in children with DCD. There was a 35% failure rate in completing this task. Children with DCD had significantly longer RTs and MTs as well as larger PF than control subjects. However, with an increase in the speed of the target, children with DCD behaved like control children to scale their movement speed and PF. Children with DCD appeared to be able to use visual-spatial information as feedback to modify their ongoing movement. Findings from the study suggest that children with DCD could benefit from interventions to improve their response time and force control and to elicit more consistent and accurate motor strategies in reaching and grasping moving targets. However, the training of speed and force modulation might not be a primary interest. Further intervention study is required to prove to support this hypothesis.
I thank Dr Louisa Law, Dr Kevin Kwong, Mr Sik-cheung Siu, and Mr Yat-man Cheung for their thoughtful comments and technical support during the development of testing instrument. I also thank the children and their parents for their participation in this study.
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developmental coordination disorder; grasp; reach; upper limb; visual feedback
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